qt send tab: rm incorrect bare raise

follow-up bc3946d2f4

not sure what the raise was trying to do;
note that relevant exception handling is done at:
bba8a272e7/electrum/gui/qt/main_window.py (L1213-L1217)
although note the TODO in main_window.on_error: would be nice to propagate some of the exceptions to the crash reporter

closes https://github.com/spesmilo/electrum/issues/8312

electrum/gui/qt/send_tab.py

@@ -272,8 +272,6 @@ class SendTab(QWidget, MessageBoxMixin, Logger):
         def sign_done(success):
             if success:
                 self.window.broadcast_or_show(tx)
-            else:
-                raise
         self.window.sign_tx(
             tx,
             callback=sign_done,